The demands on high-speed and high-definition has been increasing on electrical and electronic devices such as the ones represented by digital multifunction peripherals, or digital cameras of recent years, and high-speed transmission of large volume of digital signals has become necessary. Consequently, differential signal transmission system capable of transmitting large volume of data at high speeds has become widespread use.
In the differential signal transmission system, fundamental signals necessary for the signal transmission are transmitted to a pair of signal lines by normal-mode component in which amplitudes are substantially equal and their polarities are mutually reversed. Further, signals of harmonic wave component with frequencies of the fundamental signals are also included in the normal-mode component, in addition to the component of fundamental signals necessary for the signal transmission. However, the normal-mode component cancels the magnetic fluxes of each other out. Since the magnetic fluxes are generated by the mutual electric currents the normal-mode component can suppress radiation noise from differential transmission paths arising from the harmonic wave component.
On the other hand, in the differential signal transmission system, signals of the common-mode component with the same polarity are also transmitted to the pair of signal lines, in addition to the normal-mode component. The normal-mode component is converted into the common-mode component by the unbalanced property of the normal-mode component arising from the differential signal transmission unit, or unbalance on the pair of signal lines, resulting in generation of the common-mode component. Further, since the common-mode component allows electrical currents to flow in the same direction of each other on the pair of signal lines, and the generated magnetic fluxes are mutually strengthened, radiation noise from the differential transmission path will become large.
As a method for suppressing such a radiation noise, Japanese Patent Application Laid-Open 4-372213 discusses that radiation noise is suppressed by performing band limitation by a low pass filter (LPF) which allows a frequency spectrum necessary for data reproduction to pass the LPF at the differential signal reception unit, and removing an unnecessary spectrum with a high-frequency band.
FIG. 7 is an electrical circuit diagram illustrating a configuration of simplified LPF in Japanese Patent Application Laid-Open 4-372213. In the case of the LPF illustrated in FIG. 7, the normal-mode component reduces an amount of transmission of its harmonic wave component, by a capacitor 401, an inductor 201 and a capacitor 403, a capacitor 402, an inductor 202 and a capacitor 404 which constitute a pi type filter. On the other hand, by allowing the common-mode component to flow to a ground (GND) mainly via the capacitors 401 and 402, the amount of transmission of the common-mode component is reduced. In other words, employing such a configuration seeks to reduce the common-mode component arising from the differential signal transmission unit, and to reduce radiation noise from the pair of signal lines.
However, capacitance values of commonly available commercial capacitors, although they fall within standards, have not yet completely conformed to them. Normally they contain errors on the order of plus or minus 10%. A variation of the capacitance values of the capacitors has significant influence, when a frequency of a signal to be transmitted becomes 1 GHz or higher.
In Japanese Patent Application Laid-Open 4-372213, a variation is present in values of capacitances of the capacitors 401, 402, 403, and 404 connected to the ground (GND). When there is a variation in the values of capacitances of the capacitors 401, 402, 403, and 404, it follows that unbalance is present on the pair of signal lines. As a result, the normal-mode component will be converted into the common-mode component, and the common-mode component will be generated. Therefore, since in a conventional LPF, an amount of generation of the common-mode component is large, and reduction effect of radiation noise is low, and therefore it is necessary to suppress the generation of the common-mode component in the LPF.